Method of making resistor elements



Nov. 1, 1938. M LE S- 2,134,752

METHOD OF MAKING RESISTOR ELEMENTS Original Filed Dec. 4, 1933 INVENTOR. 5947Pg2 ff. 5/7/51 5.

A TTORNEY) Patented Nov. 1, 1938 George M. Ehlers, Milwaukee, Wis., or, by

mesne assignments, to Globe-Union Inc., waukee, Ws., a corporation of Original application December 4, 1933, Serial No. 700,872. Divided and this application October 1, 1934, Serial No. 746,340 r 4 Claims.

This invention relates to a method of making fixed resistor elements particularly applicable for radio, communication and allied electrical circuits.

The present application is a division of my co-pending application, Serial No. 700,872, filed December 4, 1933, which application is in part a continuation of and a substitute for my application for Electrical resistor elements, Serial No. 462,976, filed June 23, 1930.

The resistor element of the applications aforesaid has an electrical conducting path in the form of a core of relatively small cross-sectional area, and a rigid insulating jacket of considerably larger cross-sectional area surrounding and in immediate contact with the core whereby to increase the mechanical strength of the structure and to protect the conducting path from moisture and accidental contacts as well as to provide for the removal of heat generated in the core when in use. The resistance mixture constituting the core has substantially the same drying and firing shrinkage factor and co-efiicient of thermal expansion as the jacket. 'Ihus drying of the element before firing and cooling the element from the firing temperature in the manufacture of the element and subsequent heating and cooling of the element in service will not cause destructive stresses between the core and the jacket to crack them or disrupt their intimate contact. The core and the jacket are compounded of materials having properties for accomplishingthese results and in'addition these materials 'have the property of vitrifying into a strong integral-mass in thefiring and subsequent cooling of the element in its process of manufacture. I find that ceramic materials give these results. A ceramic material for "the core provides a vitrified insulating matrixfor the electrical conducting material of the core and holds such material evenly distributed in place in the core. A ceramic material for the jacket provides a strong ceramic insulating jacket about the, core giving the core adequate protection and support.

The object of my invention of the present application is to provide a method for making the resistor element referred to by simultaneously extruding the materials composing the core and the jacket, while in a plastic form, from a die or other apparatus so constructed that the materials as they are expressed from the die will be "simultaneously shaped to the form required for the core and the jacket and be disposed in assembled relation one within the other, and in intimate contact ready for the subsequent treatment required for the manufacture of the finished element.

This process of simultaneous extrusion of the core and the jacket materials in assembled relation is new in the art and involves careful selection of the materials for both the core and the jacket, so that they will extrude satisfactorily, will vitrify and will not crack in the drying or firing of the extruded piece. To satisfy these conditions, I find that not only the kind of materials for the core and the jacket must be correctly chosen, but also the grain sizes of the materials used is of considerable importance as hereinafter more fully set forth.

The resistor element to which my invention relates is provided with a glazed coating on the outer surface of the jacket. This coating seals the jacket against the admission of oxygen and protects the carbonaceous electrical conducting material of the. core from oxidation at the high temperature to which the piece is subjected during the period of vitrification of the jacket and the core of ceramic materials.

It is a further object of the invention of the present application to apply this coating on the outer surface of the jacket as by spraying, as the extruded piece leaves the die.

The die which constitutes the apparatus of the invention of the present application is so designed and constructed that the core and the jacket materials may be simultaneously extruded into one assembled piece.-

The invention consists further in the matters hereinafter described and claimed.

In the accompanying drawing:

Fig. 1 is a. perspective view of a resistor element constructed in accordance with the method and'apparatus of my invention;

Fig. 2 is a longitudinal sectional view of the element;

Figs. 3 and 4 are transverse sectional views taken on lines 33 and 4-4, respectively, of Fig.

Fig. 5 illustrates a modified form of terminal construction to be hereinafter described;

Fig. 6 illustrates the method and apparatus of my invention for making the element; and

Fig. I is an enlarged sectional view illustrating diagrammatically the core and jacket joined together in a vitrified integral mass.

As shown in the drawing, the resistor element constructed in accordance with my "invention, comprises a core I and a surrounding jacket 2.

The core provides an electrical conducting path 55 .cal circuit.

the ends of the Jacket by the of s table 1 1 are in the form of metal caps 3, 3 fitting overthe P The core I is in the form'of a solid rodiandfii the jacket 2 is in the iorm of a tube solid surrounding wall. 'I'lieconstructioriis'such is. in, intimate co them. The core and t gdacket are co-exte i in length so that the end adjacent the ends of the jacket, thereby ma the core accessible for connecting it in an electri Contact is made with the-"core -"at terminal meansQwhich, as shown in the'd ends of the jacket and in electrical contact with the ends of the core. These caps are preferably in the form of metal coatings applied over the ends of the jacket and the coreby a metal spray process which secures an intimate electrical contact between the core and the caps. In this form, the resistor element could be. used in an electrical circuit by inserting it between and in contact with springs or other metal contact members in the circuit.

In Figs. 1 and 2, however, I have shown lead wires 4, 4 connected to the terminal caps or coatings 3 at the ends of the element for circuit connecting purposes. The wires are preferably soldered to the caps as by dipping the ends of the element as provided by the caps in molten solder after the caps have been applied and the wires have been wound or wrapped about the caps one or more turns. This provides soldered coatings 5, 5 over the metal caps 3 to protect them, and

also secures the wires to the caps. The latter fit tightly over the ends of the Jacket and are secure against removal.

In the form of terminal construction shown in Fig. 5, the wires are directly connected with the core i, the wires at the opposite ends of the resistor element extending into the material of the core through apertures in the metal caps. Only one end of the elements is shownin Fig. 5. There the wire 4a is shown extending into the core I through an aperture 6 in the adjacent metal cap 3. The wire fits ina bore I a in the end of the core and is secured .to the metal cap by a soldering coating 5a applied over the outer side of the cap. A similar arrangement is provided at the opposite end of the element.

To give mechanicalstrength to the resistor element, the cross-sectional area of the jacket 2 is considerably larger than that of the core I. This increased wall thickness of the jacket-also protects the core from moisture and accidental electrical contacts and additionally facilitates the removal of the heat generated in'the core in the "ime'sia jataaifies thereto to provide an integral connectionzbetween and the jacket to be vitrified into a strong integral mass in the firing and subsequent cooling of the extruded piece to give the element the desired mechanical strength, thermal shock resistance and thermal conductivity the desired resistance value after firing; to provide the core :lin the form of a vitrified matrix to hold the eleciced.w conductin andto obtain the 7 eating and cooling of the eles of manufacture and also when It is to be understood that the ma- IL SFY Q -zer1a1- or the core in its unfinished state before ing. may; not be :at iallpelectri'cally conducting, t becomes conducting-Rafter; :being i fired. 1.'= The truded rods even.-in .;their dried? state before firirig have a very much higher electrical resistance than tl' ey'have in the.-finish e d firedsstate-yfisj' Thepurpose; of the relatively; small diameter core herein mentioned ls .thatsmore. desirableelectrical characteristicscan be scontrollablyiobtained with a core having a high proportion .of:.conducting material in its composition.: Since this 'gives i the composition a low specific resistancafiit is necessary to provide a core of relatively small cross sectional area to get the desired high-resistance values in the finished resistors.

Certain ceramic materials have the properties I which give these results. The ceramic materials selected for the core need not necessarily be the same as the ceramic materials selected for the jacket, but it is essential that the core and the jacket both contain ceramic materials to obtain their vitrification and the matching or equalization of the two bodies in shrinkage and thermal co-efilcients of expansion. It is also essential to maintain a certain grain size relation between the materials composing the core and those composing the jacket. It was found that a core having the desired characteristics is composed of a relatively coarse grain, non-plastic, refractory ceramic (approximately 100 mesh), a fine grain plastic ceramic, sufiiciently fine to make a plastic colloidal suspension, and an electric conducting material such as finely divided graphite or carbon. It was found that a Jacket having the desired characteristics is composed of a relatively fine grain size, non-plastic refractory ceramic (approximately 200 mesh or finer), fine plastic ceramic clays, and a quantity of some refractory material of good thermal, conductivity. The finely divided ceramics in the core and the jacket render: their mixtures sufilciently plastic to facilitate their extrusion in the making of the element, as well as serving as binders for the nonplastic ingredients of the core and the jacket, namely, the non-plastic ingredients in the jacket and the refractory ceramic and the conducting material in the core. 'The ceramic mixture of the jacket is much more complex than the ceramic mixture of the core. The purpose of this complex mixture is to obtain a vitrified body, which will have the desired mechanical strength, thermal heat shock resistance and thermal conductivity. The choice of these ceramic materials, of

core material than in the jacket material is lm-.

arse-15a portant in obtaining the proper drying'shrinkage 'of-the-core relatively to the jacket. Theinclua any particular or designated ceramics or other except as herein claimed, it. being withsion of the mic plastic ceramicsin the core and the jacket materials renders" them sufficiently plastic for extruding these materials in the makingof the element. The ceramics in the two hodiesjuse together in the making of the element.

be these best smtedtoaccomplish the results herein set forth. Ido not wish to be'limited'to in the contemplation andscope of my invention make the resistor element of these which will give the results herein described and will provide theideslredrmatrices.

The-method of and apparatus for making-the resistor element as shown in figfii'enables the core I and the jacket 2 to be produced simul- "tan'eously in'their assembled relation, onewlththe desired plastic mixture for the jacket. These materials are then subjected to pressure, as by pistons or plungers H, H operating in the respective cylinders and the contents of the cylinders are forced out of the same through the nozzles in the form of a continuous extruded piece or rod IS, the core material being within the jacket material and in intimate contact therewith, as shown. The rod has the shape required for the core and the jacket for the finished element.

As the rod I3 is extruded from the nozzle, it has been found desirable to coat the outer surface of the rod with a coating M of a glaze enamel having a lower melting point than the vitrifying temperature of the ceramic materials of the core and the jacket. This is not an indispensable coating, but has been found desirable as hereinafter explained. The enamel coating ll, called a vitreous enamel frit, may be applied to the outer surface of the rod as it is extruded from the nozzles in any desired manner. A spraying process is found desirable for this pur pose, spray nozzles I5, I 5 of the character required being used, as illustrated in Fig. 6.

These nozzles are so positioned and arranged about the path of the rod as it is extruded from the die that an enamel coating is applied simultaneously about the entire outer surface of the rod asit leaves the die. Three nozzles may be used. They are disposed in substantially right angular relation to the axis of the extruded piece and in equal spaced relation about the same. This arrangement as shown in Fig. 6 is by way of an example, it being understood that the nozzles will be so arranged and disposed to coat the entire outer surface of the rod with the enamel coating l4 simultaneously and as the rod leaves the forming die.

The extruded and enamel coated rods iii are cut into lengths suitable for drying and firing. After drying in a suitable oven, the rods are passed through a furnace at a temperature sufllcient to drive of! volatile material and water without melting the enamel coating into a glaze and without vitrifying the jacket material.

The temperatures to which the rods have been subjected up to this part of the process are not coating of low melting point glaze. :Iamics suitable for the jacket and having'the desired-mechanical strength and heat shock re- .degree' of-lomdation .of the core.

- afurnace at a temperature sufliciently'high-to vitrifv and seal. the materials of the protecting jacket. The enamel frit on the surface-of the rods forms a glaze which seals the jacket against the admission of oxygen and protects the carbonaceous core. from oxidation during the period "of'vitrification of theijacket. andcoreiceramic When 'fthe jacket material itself is chosen vsothat its pores are sealed-almost as quickly as the rod is subjected to-the elevated temperature, then it is unnecessary to apply the Most cesistance after. firing =do..not, however, seal rapidly enough at the high temperature to protect the core material .from. oxidation. This results in a great non-uniformity ofresistance value. for the finished resistor 'due to variations .in the It has, therefore, been found .dmirable in my process of manufacture of resistors toprovide'the above mentioned low melting glaze on the surface of the resistor. The presence of the glaze on the finished resistor alsoserves to protect the core from the penetration of water vapor which would cause changes in the resistance value.

As the high temperature of the furnace penetrates the core material, the ceramic contained therein is also vitrified, with the result that the core is vitrified in intimate contact with the surrounding ceramic jacket. The vitrification of the ceramic material incorporated in the core, binds the conducting particles firmly into place whereby a permanent and stable resistance is formed.

The process so described results in the manufacture of a mechanically strong, hard ceramic rod containing a conducting core. These rods are then cut into desired lengths, and the ends are provided with terminals as heretofore described- The above described method of making an electrical resistor element results in superior electrical characteristics of the resistance, as well as in advantages of mechanical strength and protection from outside contact and humidity. These superior electrical characteristics lie in its constant value of resistance. Many resistors made of a mixture of carbon and insulating material. depend for the resistance value obtained upon the contact resistance between adjacent particles of carbon. Such resistances manifest what is commonly called a high voltage co-eiiicient, whereas the voltage co-eflicient of the resistor described herein is relatively very low. By this term, "voltage co-efflcient, is meant the percentage change of resistance for a given change in voltage. In changing the voltage applied to the resistor, many of the existing types of resistances show an excessive change in varying the voltage from a value of the order' of 10 volts to a value of the order of 300 volts. In the resistors of my invention, this change of resistance with voltage is of the order of less than 10% for a similar change in voltage. My improved resistor also minimizes the so-cailed microphonic noise frequently occurring when composition resistors of the prior art are connected into an amplifying circuit whose output feeds a sound reproducing device. Freedom from change due to atmospheric humidity is also a characteristic of my new resistance.

The details of structure shown and described may be variously changed and modified without departing from the spirit and scope of my invention.

I claim as my invention:

1. The method of making an electrical resistor element which includes embedding a citrifiable material containing a conductor into a citriflable material which is in a state of greater sub-division than the first material and firing the formed materials to a state of vitrification.

2. The hereindescribed method of making a rod-like electric resistor which consists in separately preparing an electric conductive core mixture made up of a coarse grained non-plastic refractory ceramic, a fine grained plastic ceramic and an electric conducting material, and an electrically insulating jacket mixture made up of fine grained non-plastic refractory ceramic, fine plastic clays and refractory material of good thermal conductivity, simultaneously extruding said core and jacket mixtures through concentric nozzles to produce the rod-like resistor with the jacket in the form of an open ended tube surrounding the core, anddrying and firing the ex- CERTIFICATE Patent No. 2,132+,752.

, jacket.

- "or sit? ifiiiifiiil-l ii o n? Siifii'ifZ-Q truded rod: to vitr itg migrating, material throughout to obtain a pennangrrt ntegralgand homogeneous bonding of the jacket ang; cage.

3. The method or making rod-like ceramic eleceg trical resistor elements which comprises enveloping a core body ofaigranular ceramic mixture including electrical conducting material with a jacket of non-conducting ceramic mixture including granular ceramic material which is in a state of greater subdivision than the ceramic material of the core mixture and firing the jacketed core to vitrify the same.

4. In the art of making ceramic electrical resistor elements including a ceramic core containing electrical conducting material surrounded by a non-conducting jacket of ceramic material, the method of increasing the bond between the core and jacket comprising including in the materials forming the core and jacket granular ceramic material, the grain size of the ceramic material included in the core being larger than the grain size of the ceramic material included in the GEORGE M. Emma.

OF CORRECTION.

November 1, 1958.

GEORGE M. EHLERS. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows column, line 8, claim 1, for

: Page 1 first "citrifiable" read vitrifiable; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent office.

Signed and sealed this 13th day of December, A. D. 1938.

(Seal) Henry Van Arsdale Acting Commissioner of Patents. 

